1. Field of the Invention
This invention is related to power supply-modulator circuits for amplitude modulated radio transmitters.
2. Description of the Related Art
Prior amplifier circuits, for example, U.S. Pat. No. 4,403,197, issued to Swanson, employs digital switching of incremental signal sources for amplifying an amplitude and frequency varying input signal to high power levels. Each of the switches has two states. In the first state, the associated incremental voltage source is interconnected in series with the other selected voltage sources across the output, whereas in the other state, the associated incremental voltage source is disconnected from the output.
In the implementation of such series switching power supply-modulator circuits for amplitude modulation radio transmitters, a recurring problem is that when the switching algorithm requires that one of the switching modules near the grounded end of the series stack turn on or off, this produces a step function change which is added to the voltage to ground of all of the modules above it. This in turn produces large transient capacitive currents, since the secondary windings of the power transformers in these circuits unavoidably have significant capacitance to ground (the electrostatic shield between the primary and secondaries).
U.S. Pat. No. 4,745,368 issued to Lodahl utilizes current limiters to limit the high peak current of the semi-conductor switches caused by the commutation characteristics of the cascade-connected semiconductor diodes. Lodahl further utilizes linearization devices compensating for non-linearities in the transmission characteristic of an amplifier. Although Lodahl deals with limiting transient currents and improving the linearity of the switching amplifier, it does not provide a means for equalizing the capacitive currents for each module. Thus, the capacitive currents are still large for a module near the ground end of the series stack of switches and much smaller for a module near the top of the series stack of switches. The variation in capacitive current is also present in other such switchable power supply-modulated radio transmitters as illustrated by the disclosures of Furrer (U.S. Pat. No. 4,560,944) and Beeken et al (U.S. Pat. No. 4,730,167). This variation in the capacitive current tends to produce variation in switching times and, as a result, distortion of the output audio waveform.
Also, the switching sequence of the modules in Furrer and Beeken et al use a simple inchworm sequence in which modules are turned on in the order of their position in the stack, beginning at the bottom. Since adjacent modules are being turned on, transient voltages from several successively switched modules are produced and added due to the close electrical and physical proximity between the successively switched modules. This causes unequal loading.